JP3654005B2

JP3654005B2 - Method for producing positive electrode plate for lithium ion secondary battery

Info

Publication number: JP3654005B2
Application number: JP26409298A
Authority: JP
Inventors: 賢治中井; 学落田; 克典鈴木
Original assignee: Shin Kobe Electric Machinery Co Ltd
Current assignee: Resonac Corp
Priority date: 1998-09-18
Filing date: 1998-09-18
Publication date: 2005-06-02
Anticipated expiration: 2018-09-18
Also published as: JP2000090917A

Description

【０００１】
【発明の属する技術分野】
本発明はリチウムイオン二次電池用正極板の製造方法に関するものである。
【０００２】
【従来の技術】
ＬｉＣｏＯ₂等からなるリチウム複合酸化物を正極材として用いる正極板と、リチウムイオンを吸蔵、放出する炭素材料等を負極材として用いる負極板とが非水電解質を介して積層されたリチウムイオン二次電池が知られている。一般にリチウムイオン二次電池の正極板は、前述のリチウム複合酸化物（正極材）と、バインダと、有機溶媒とを含む正極材スラリーを集電体上に塗布してからこの正極材スラリーを乾燥して製造する。リチウムイオン二次電池は、エネルギー密度が高く、主にＶＴＲカメラ，ノートパソコン，携帯電話等のポータブル機器に使用されている。最近では、このリチウムイオン二次電池のエネルギー密度を更に高めることが求められている。そこでＬｉＣｏＯ₂に代えて、ＬｉＮｉＯ₂，ＬｉＮｉx Ｃｏy Ｏ₂等の少なくともリチウム元素及びニッケル元素を含むリチウムニッケル複合酸化物を主成分とする正極材を用いるリチウムイオン二次電池が開発された。リチウムニッケル複合酸化物の単位重量あたりの容量（１８０〜２００ mAh/g）は、ＬｉＣｏＯ₂の単位重量あたりの容量（１４５〜１５０ mAh/g）に比べて大幅に高い。
【０００３】
【発明が解決しようとする課題】
しかしながら、このようなリチウムニッケル複合酸化物を用いて正極材スラリーを作ると正極材スラリーが流動性を失いゲル化するという問題が生じる。これは次のような理由によると考えられる。他のリチウム含有複合酸化物に比べて、少なくともリチウム元素及びニッケル元素を含むリチウムニッケル複合酸化物は、生成された段階において、Ｌｉ₂Ｏ等のフリーのリチウムが不純物として残留しやすい。このＬｉ₂Ｏがスラリー中に微量に含まれる水分と反応すると、Ｌｉ₂Ｏ＋Ｈ₂Ｏ→２ＬｉＯＨの反応式で水酸化リチウム（ＬｉＯＨ）を生成し、スラリーが比較的強いアルカリ性になる。例えば、この種のリチウムニッケル複合酸化物を水に対して約１１重量％分散させて分散液を作ると、分散液のｐＨは約１２になる。これにより、スラリー中のバインダが三次元化して正極材スラリーがゲル化すると考えられる。特にバインダとして、ポリフッ化ビニリデン（ＰＶＤＦ）を用い、有機溶媒としてＮ−メチル−２−ピロリドン（ＮＭＰ）を用いるとバインダの三次元化が著しい。そこで、正極材スラリー生成時に除湿等の雰囲気対策を講じることも考えられるが、このような対策を講じると、製造コストが高くなる。また、このような対策を講じても活物質中にＬｉ₂Ｏが存在するので、活物質中にＬｉＯＨやＬｉ₂ＣＯ₃が生成され、電池の充放電が阻害されて高負荷時の放電容量が低くなる。
【０００４】
本発明の目的は、製造コストを高めたり、電池の高負荷時の放電容量を低下させることなく、正極材スラリーがゲル化するのを防止できるリチウムイオン二次電池用正極板の製造方法を提供することにある。
【０００５】
【課題を解決するための手段】
本発明は、少なくともリチウム元素及びニッケル元素を含むリチウムニッケル複合酸化物を主成分とする正極材と、バインダと、有機溶媒とを含むスラリーを集電体上に塗布してリチウムイオン二次電池用正極板を製造する方法を改良の対象にする。ここでいう、少なくともリチウム元素及びニッケル元素を含むリチウムニッケル複合酸化物とは、ＬｉＮｉＯ₂のようにＬｉ及びＮｉを含むリチウムニッケル複合酸化物、またはＬｉＮｉx Ｃｏy Ｏ₂（x ＋y ＝１）のようにＬｉ及びＮｉに加えて、Ｌｉ及びＮｉ以外の元素も少なくとも１つ有するリチウムニッケル複合酸化物である。本発明では、正極材を水に対して８〜１２重量％分散させて分散液を作った場合に、分散液のｐＨが７．１〜１１．２になるものを正極材として用いる。
【０００６】
このようにして水に分散しても強いアルカリ性にならないリチウムニッケル複合酸化物を作れば、正極材スラリーに微量に水分が含まれていても、正極材スラリーは、ほぼ中性または弱アルカリ性になり、バインダの三次元化を抑制して、正極材スラリーがゲル化するのを防ぐことができる。なお、分散液のｐＨが７．１を下回ると、活物質が電解液に溶解しやすくなるため、放電容量が低くなる。このような正極材は、種々の方法で生成できる。例えば、Ｌｉ₂Ｏとリチウムニッケル複合酸化物とを含むリチウムニッケル複合酸化物材料を水に対して８〜１２重量％分散させて分散液を作り、この分散液のｐＨが７．１〜１１．２になるまで分散液を撹拌する。次にこの分散液を濾過して、残渣を取出してから、乾燥して正極材を生成する。このようにすれば、分散液を撹拌する際に空気中のＣＯ₂が分散液に溶け込んでＨ₂ＣＯ₃になる。このＨ₂ＣＯ₃は次の式により、Ｌｉ₂Ｏと水とが反応して生成された分散液中のＬｉＯＨと反応する。
【０００７】
【化１】

これにより、分散液のｐＨは小さくなり、分散液が中性化する。この時のｐＨは、分散液の撹拌速度または撹拌時間等により調整すればよい。
【０００８】
本方法で製造された正極板の正極材は、上記式で生成されたＬｉ₂ＣＯ₃が分散液濾過後に残留することになる。即ち、リチウムニッケル複合酸化物に加えてＬｉ₂ＣＯ₃を含むことになる。
【０００９】
また他の方法では、Ｌｉ₂Ｏとリチウムニッケル複合酸化物とを含むリチウムニッケル複合酸化物材料を水に対して８〜１２重量％分散させて分散液を作り、分散液のｐＨが７．１〜１１．２になるまで分散液に燐酸（Ｈ₃ＰＯ₄）を加える。次にこの分散液を濾過して、残渣を取出してから、乾燥して正極材を生成する。このようにすれば、Ｈ₃ＰＯ₄は次の式によりＬｉ₂Ｏと水とが反応して生成された分散液中のＬｉＯＨと反応する。
【００１０】
【化２】

これにより、分散液のｐＨは小さくなり、分散液が中性化する。
【００１１】
なお、本発明者が試験したところ、分散液のｐＨを低下させるために、塩酸、硫酸、硝酸、硼酸、塩素酸等の燐酸以外の無機酸または有機酸を用いても十分な放電容量を得ることはできなかった。
【００１２】
本方法で製造された正極板の正極材は、上記式で生成されたＬｉ₃ＰＯ₄が分散液濾過後に残留することになる。即ち、リチウムニッケル複合酸化物に加えてＬｉ₃ＰＯ₄を含むことになる。
【００１３】
前述したように、バインダとしてポリフッ化ビニリデン（ＰＶＤＦ）を用い、有機溶媒としてＮ−メチル−２−ピロリドン（ＮＭＰ）を用いた場合に、スラリーが比較的強いアルカリ性になった際のバインダの三次元化が著しい。そこで、このようなバインダ（ＰＶＤＦ）及び有機溶媒（ＮＭＰ）を用いた場合には、本発明の効果は著しく高くなる。
【００１４】
【発明の実施の形態】
試験に用いた各リチウムイオン二次電池用正極板の正極材スラリーを次のようにして作った。まず、Ｌｉ₂ＯとＬｉＮｉ_0.8Ｃｏ_0.2Ｏ₂とを含むリチウムニッケル複合酸化物材料５ｇを水４５ｇに分散して（リチウムニッケル複合酸化物材料を水に対して１１重量％分散させて）分散液を作る。本例では、本莊ＦＭＣエナジー株式会社から販売されているリチウムニッケル複合酸化物材料を用いた。このリチウムニッケル複合酸化物材料に含まれるＬｉ₂Ｏの量は定かではない。しかしながら、下記の表１の比較例１に示される撹拌時間が０分のときのｐＨ値は、リチウムニッケル複合酸化物材料に含まれるＬｉ₂Ｏの量に比例している。即ち、Ｌｉ₂Ｏの含有量が多ければ撹拌時間が０分のときのｐＨ値は大きくなり、Ｌｉ₂Ｏの含有量が少ないときにはこのｐＨ値は小さくなる。したがってこのｐＨ値よりＬｉ₂Ｏの含有量は、計算によりある程度求めることは可能である。Ｌｉ₂Ｏの含有量が不明であっても、撹拌時間によってｐＨ値の調整ができるため、Ｌｉ₂Ｏの含有量は不明であっても問題にはならない。次にマグネットスターラーを用いて表１に示す時間の撹拌を分散液にそれぞれ行って、ｐＨ値の異なる各分散液を作った。このように分散液を撹拌すると空気中のＣＯ₂が分散液に溶け込む。そのため、撹拌時間が長いほど分散液のｐＨ値は小さくなる。なお、本試験では、ｐＨ測定器を汚さないために、分散液を撹拌後６０分間静止してから、分散液の上澄液のｐＨ値を測定した。次に分散液をそれぞれ濾過してリチウムニッケル複合酸化物を取出した。次にこれらを９０℃の温度で２４時間加熱して乾燥し、平均粒径２０μｍの各リチウムニッケル複合酸化物を得た。
【００１５】
また、これらのリチウムニッケル複合酸化物とは別にＬｉ₂ＯとＬｉＮｉ_0.8Ｃｏ_0.2Ｏ₂とを含むリチウムニッケル複合酸化物５ｇを水４５ｇに分散して（リチウムニッケル複合酸化物材料を水に対して１１重量％分散させて）分散液を作った。次に濃度８．５重量％の燐酸（Ｈ₃ＰＯ₄）を表２に示す量だけ分散液にそれぞれ滴下してから、マグネットスターラーを用いて５分間撹拌した。そして、分散液を６０分間静止して、分散液の上澄液のｐＨ値を測定した。次に分散液をそれぞれ濾過してリチウムニッケル複合酸化物を取出した。次にこれらを９０℃の温度で２４時間加熱して乾燥し、平均粒径２０μｍの各リチウムニッケル複合酸化物を得た。
【００１６】
次に上記各リチウムニッケル複合酸化物８０重量％を、平均粒径０．５μｍの黒鉛からなる導電剤１０重量％と、ポリフッ化ビニリデン（ＰＶＤＦ）からなるバインダ１０重量％とにそれぞれ混合した。次にこれにＮ−メチル−２−ピロリドン（ＮＭＰ）からなる有機溶媒を適量加えて十分に混練して各正極材スラリーを作った。なお、正極材スラリー作成時の雰囲気は５０％ＲＨとした。次に２０μｍ×５０ｍｍ×４５０ｍｍの帯状のアルミニウム箔からなる正極集電体の両面にロールトゥロール転写により正極材スラリーを塗布してから乾燥、プレスして両面にそれぞれ厚み８０μｍの正極材層を形成して、表１及び２に示す比較例１Ａ，１Ｂ及び実施例１Ａ〜１Ｅ並びに比較例２Ａ〜２Ｃ及び実施例２Ａ〜２Ｄの各正極板を作った。表１及び２には、各正極板の製造時の正極材スラリーの流動性の有無が示されている。
【００１７】
【表１】

【表２】

表１及び２より分散液のｐＨ値が１１．２を上回る比較例１Ａ，１Ｂ，２Ａ，２Ｂでは、正極材スラリーの流動性がなく電池として有効に機能する正極板を形成することができなかった。
【００１８】
次に比較例１Ａ，１Ｂ，２Ａ，２Ｂを除く各正極板を用いて図１に示す試験用のリチウムイオン二次電池を作った。本図に示すように、リチウムイオン二次電池は、巻回式極板群１が電池缶２内に収納された構造を有している。そして、巻回式極板群１は、正極板３と負極板４とが電解質層（セパレータ）５を介して積層するように巻回された構造を有している。正極板３は正極集電体６の両面に正極材層７が形成された構造を有している。また、負極板４は負極集電体８の両面に負極材層９が形成された構造を有している。リチウムイオン二次電池は次のように製造した。まず、負極板４を製造した。最初に、平均粒子径１５μｍの黒鉛の炭素材料からなる負極材９０重量％と、ポリフッ化ビニリデンからなるバインダ１０重量％とを混合した。なお、負極材としては非晶質炭素を用いることもできる。これにＮ−メチル−２−ピロリドン（ＮＭＰ）からなる溶媒を適量加えて十分に混練して負極スラリーを作った。次に１０μｍ×５０ｍｍ×４９０ｍｍの帯状の銅箔からなる負極集電体８の両面にロールトゥロール転写により負極材スラリーを塗布してから乾燥、プレスして両面にそれぞれ厚み１０５μｍの負極材層９を形成して負板４を作った。なお、この負極材層９の密度は１．３〜１．４５ｇ／ｃｍ³である。
【００１９】
次に、前述の各正極板３と負極板４とを厚み２５μｍのポリエチレン微多孔膜からなる帯状のセパレータ５を介してそれぞれ巻回して極板群１を作った。次に、極板群１をＮｉめっき鉄からなる円筒形の電池缶２内に配置してから、予め負極集電体８に溶接してあるニッケルタブ端子１１を電池缶２の底部２ａに溶接した。次に炭酸エチレンと炭酸ジメチルと炭酸ジエチルとを体積比３０：５０：２０で混合した溶媒にＬｉＰＦ₆からなるリチウム塩を１モル／ｌの濃度で溶解した有機電解液（非水電解液）を電池缶２内に５ｍｌ注入した。次に予め正極集電体６に溶接してあるアルミニウムタブ端子１０を電流遮断機構（圧力スイッチ）及び弁（図示せず）を備える電池蓋１２に溶接した。なお、この弁は電流遮断機構（圧力スイッチ）が作動する圧力より高い圧力で開放作動を行う。そして、電池蓋１２を絶縁性のポリプロピレンからなるガスケット１３を介して電池缶２の上部に配置してから、これをかしめて電池缶２内を密閉して円筒形の各リチウムイオン二次電池を作った。
【００２０】
次に各リチウムイオン二次電池を２５℃の雰囲気中において４．２１Ｖの低電圧（制限電流３２０ｍＡｈ）で８時間充電してから、１．６Ａで終止電圧２．５Ｖまで放電して正極材（リチウムニッケル複合酸化物）１ｇあたりの放電容量を求めた。表１及び２には、各正極板を用いて作った電池の放電容量が示されている。表２よりｐＨ値が７．１を下回る比較例２Ｃでは、活物質が電解液に溶解するため、放電容量が低くなるのが分かる。
【００２１】
次に、表３に示すように、分散液の撹拌時間または燐酸の添加量を変えて、ｐＨ値が１１．２を上回る値でそれぞれ異なるリチウムニッケル複合酸化物を得た。そして、各リチウムニッケル複合酸化物を用いて、除湿装置による３％ＲＨの雰囲気中で正極材スラリー作成し、リチウムイオン二次電池を作った。なお、正極材スラリー及びリチウムイオン二次電池は、雰囲気の湿度を除いては、前述の試験と同じ条件で製造した。そして、正極材スラリーの流動性の有無及びリチウムイオン二次電池の正極材（リチウムニッケル複合酸化物）１ｇあたりの放電容量を調べた。表３はその測定結果を示している。
【００２２】
【表３】

表３より、３％ＲＨの雰囲気中で作成した各正極材スラリーは流動性を有しているのが分かる。しかしながら、各正極材スラリーを用いて製造した電池は、高負荷時の放電容量が１５４〜１５８ｍＡｈ／ｇと低く、十分な容量を得ることができないのが分る。したがって、正極材スラリー生成時に除湿の雰囲気対策を講じても、高負荷時の放電容量は低いままであるのが分る。
【００２３】
【発明の効果】
本発明によれば、水に分散しても強いアルカリ性にならないリチウムニッケル複合酸化物を用いて正極材スラリーを作るので、正極材スラリーに微量に水分が含まれていても、正極材スラリーは、ほぼ中性または弱アルカリ性になり、バインダの三次元化を抑制して、正極材スラリーがゲル化するのを防ぐことができる。
【図面の簡単な説明】
【図１】本発明の実施例の方法で製造したリチウムイオン二次電池の端面図である。
【符号の説明】
１巻回式極板群
２電池缶
３正極板
４負極板
５電解質層（セパレータ）
６正極集電体
７正極材層
８負極集電体
９負極材層[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a positive electrode plate for a lithium ion secondary battery.
[0002]
[Prior art]
A lithium ion secondary in which a positive electrode plate using a lithium composite oxide made of LiCoO ₂ or the like as a positive electrode material and a negative electrode plate using a carbon material or the like that absorbs and releases lithium ions as a negative electrode material are stacked via a non-aqueous electrolyte. Batteries are known. In general, a positive electrode plate of a lithium ion secondary battery is formed by applying a positive electrode material slurry containing the above-described lithium composite oxide (positive electrode material), a binder, and an organic solvent on a current collector, and then drying the positive electrode material slurry. To manufacture. Lithium ion secondary batteries have high energy density and are mainly used in portable devices such as VTR cameras, notebook computers, and mobile phones. Recently, it has been demanded to further increase the energy density of the lithium ion secondary battery. Accordingly, a lithium ion secondary battery using a positive electrode material mainly composed of a lithium nickel composite oxide containing at least a lithium element and a nickel element such as LiNiO ₂ and LiNix Coy O _{2 has been} developed instead of LiCoO ₂ . The capacity per unit weight (180 to 200 mAh / g) of the lithium nickel composite oxide is significantly higher than the capacity per unit weight (145 to 150 mAh / g) of LiCoO ₂ .
[0003]
[Problems to be solved by the invention]
However, when a positive electrode material slurry is made using such a lithium nickel composite oxide, there arises a problem that the positive electrode material slurry loses fluidity and gels. This is considered to be due to the following reasons. Compared to other lithium-containing composite oxides, in the lithium nickel composite oxide containing at least lithium element and nickel element, free lithium such as Li ₂ O tends to remain as impurities in the generated stage. When this Li ₂ O reacts with moisture contained in a minute amount in the slurry, lithium hydroxide (LiOH) is generated by a reaction formula of Li ₂ O + H ₂ O → 2LiOH, and the slurry becomes relatively strong alkaline. For example, when this type of lithium nickel composite oxide is dispersed in water by about 11% by weight to make a dispersion, the pH of the dispersion becomes about 12. Thereby, it is considered that the binder in the slurry becomes three-dimensional and the positive electrode material slurry gels. In particular, when polyvinylidene fluoride (PVDF) is used as the binder and N-methyl-2-pyrrolidone (NMP) is used as the organic solvent, the binder becomes three-dimensional. Therefore, it is conceivable to take measures against the atmosphere such as dehumidification when the positive electrode material slurry is generated. However, if such measures are taken, the manufacturing cost increases. Even if such measures are taken, since Li ₂ O is present in the active material, LiOH and Li ₂ CO ₃ are generated in the active material, and charging / discharging of the battery is inhibited, resulting in a discharge capacity at high load. Becomes lower.
[0004]
An object of the present invention is to provide a method for producing a positive electrode plate for a lithium ion secondary battery that can prevent the positive electrode material slurry from gelling without increasing the production cost or reducing the discharge capacity at high load of the battery. There is to do.
[0005]
[Means for Solving the Problems]
The present invention relates to a lithium ion secondary battery by applying a slurry containing a positive electrode material mainly composed of a lithium nickel composite oxide containing at least lithium element and nickel element, a binder, and an organic solvent on a current collector. The method of manufacturing the positive electrode plate is targeted for improvement. Here, the lithium nickel composite oxide containing at least lithium element and nickel element is a lithium nickel composite oxide containing Li and Ni, such as LiNiO ₂ , or LiNix Coy O ₂ (x + y = 1). In addition to Li and Ni, it is a lithium nickel composite oxide having at least one element other than Li and Ni. In the present invention, when the dispersion is made by dispersing 8 to 12% by weight of the positive electrode material with respect to water, a material having a pH of 7.1 to 11.2 is used as the positive electrode material.
[0006]
If a lithium nickel composite oxide that does not become strong alkalinity even when dispersed in water in this way, the positive electrode material slurry becomes almost neutral or weakly alkaline even if the positive electrode material slurry contains a small amount of water. Further, the three-dimensionalization of the binder can be suppressed and the positive electrode material slurry can be prevented from gelling. Note that when the pH of the dispersion is lower than 7.1, the active material is easily dissolved in the electrolytic solution, so that the discharge capacity is lowered. Such a positive electrode material can be produced by various methods. For example, a lithium-nickel composite oxide material containing Li ₂ O and lithium-nickel composite oxide is dispersed in water by 8 to 12% by weight to make a dispersion, and the pH of the dispersion is 7.1 to 11. Stir the dispersion until 2. Next, this dispersion is filtered to remove the residue, and then dried to produce a positive electrode material. Thus, CO ₂ in the air is H ₂ CO ₃ dissolved in the dispersion at the time of stirring the dispersion. This H ₂ CO ₃ reacts with LiOH in the dispersion produced by the reaction of Li ₂ O and water according to the following equation.
[0007]
[Chemical 1]

Thereby, pH of a dispersion liquid becomes small and a dispersion liquid becomes neutral. The pH at this time may be adjusted by the stirring speed or stirring time of the dispersion.
[0008]
In the positive electrode material of the positive electrode plate manufactured by this method, Li ₂ CO ₃ produced by the above formula remains after the dispersion liquid filtration. That is, Li ₂ CO ₃ is included in addition to the lithium nickel composite oxide.
[0009]
In another method, a lithium nickel composite oxide material containing Li ₂ O and a lithium nickel composite oxide is dispersed in an amount of 8 to 12% by weight with respect to water to form a dispersion, and the pH of the dispersion is 7.1. until ～11.2 added phosphoric acid (H ₃ PO ₄₎ to the dispersion. Next, this dispersion is filtered to remove the residue, and then dried to produce a positive electrode material. In this way, H ₃ PO ₄ reacts with LiOH in the dispersion produced by the reaction of Li ₂ O and water according to the following formula.
[0010]
[Chemical formula 2]

Thereby, pH of a dispersion liquid becomes small and a dispersion liquid becomes neutral.
[0011]
In addition, as a result of testing by the present inventor, sufficient discharge capacity can be obtained even if inorganic acid or organic acid other than phosphoric acid such as hydrochloric acid, sulfuric acid, nitric acid, boric acid, chloric acid is used to lower the pH of the dispersion. I couldn't.
[0012]
In the positive electrode material of the positive electrode plate manufactured by this method, Li ₃ PO ₄ produced by the above formula remains after the dispersion filtration. That is, Li ₃ PO ₄ is contained in addition to the lithium nickel composite oxide.
[0013]
As described above, when polyvinylidene fluoride (PVDF) is used as the binder and N-methyl-2-pyrrolidone (NMP) is used as the organic solvent, the three-dimensional of the binder when the slurry becomes relatively strong alkaline. It is remarkable. Therefore, when such a binder (PVDF) and an organic solvent (NMP) are used, the effect of the present invention is remarkably enhanced.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
A positive electrode material slurry of each positive electrode plate for a lithium ion secondary battery used in the test was prepared as follows. First, 5 g of lithium nickel composite oxide material containing Li ₂ O and LiNi _0.8 Co _0.2 O ₂ is dispersed in 45 g of water (the lithium nickel composite oxide material is dispersed by 11 wt% with respect to water). make. In this example, a lithium nickel composite oxide material sold by Honjo FMC Energy Co., Ltd. was used. The amount of Li ₂ O contained in this lithium nickel composite oxide material is not certain. However, the pH value when the stirring time shown in Comparative Example 1 in Table 1 below is 0 minutes is proportional to the amount of Li ₂ O contained in the lithium nickel composite oxide material. That is, if the content of Li ₂ O is large, the pH value when the stirring time is 0 minute becomes large, and this pH value becomes small when the content of Li ₂ O is small. Therefore, the Li ₂ O content can be determined to some extent by calculation from this pH value. Even if the content of Li ₂ O is unknown, the pH value can be adjusted by the stirring time. Therefore, even if the content of Li ₂ O is unknown, there is no problem. Next, each dispersion was stirred for the time shown in Table 1 using a magnetic stirrer to prepare dispersions having different pH values. When the dispersion is stirred in this way, CO ₂ in the air dissolves in the dispersion. Therefore, the longer the stirring time, the smaller the pH value of the dispersion. In this test, in order not to soil the pH measuring device, the dispersion was allowed to stand for 60 minutes after stirring, and then the pH value of the supernatant of the dispersion was measured. Next, each of the dispersions was filtered to extract a lithium nickel composite oxide. Next, these were heated at a temperature of 90 ° C. for 24 hours and dried to obtain lithium nickel composite oxides having an average particle diameter of 20 μm.
[0015]
In addition to these lithium nickel composite oxides, 5 g of lithium nickel composite oxide containing Li ₂ O and LiNi _0.8 Co _0.2 O ₂ was dispersed in 45 g of water (the lithium nickel composite oxide material was dissolved in water). A dispersion was made (11% by weight dispersed). Next, phosphoric acid (H ₃ PO ₄ ) having a concentration of 8.5% by weight was dropped into the dispersion by the amount shown in Table 2, and then stirred for 5 minutes using a magnetic stirrer. Then, the dispersion was allowed to stand for 60 minutes, and the pH value of the supernatant of the dispersion was measured. Next, each of the dispersions was filtered to extract a lithium nickel composite oxide. Next, these were heated at a temperature of 90 ° C. for 24 hours and dried to obtain lithium nickel composite oxides having an average particle diameter of 20 μm.
[0016]
Next, 80% by weight of each lithium nickel composite oxide was mixed with 10% by weight of a conductive agent made of graphite having an average particle diameter of 0.5 μm and 10% by weight of a binder made of polyvinylidene fluoride (PVDF). Next, an appropriate amount of an organic solvent composed of N-methyl-2-pyrrolidone (NMP) was added thereto and sufficiently kneaded to prepare each positive electrode material slurry. The atmosphere at the time of preparing the positive electrode material slurry was 50% RH. Next, apply a positive electrode material slurry by roll-to-roll transfer on both sides of a positive electrode current collector made of a 20 μm × 50 mm × 450 mm strip-shaped aluminum foil, and then dry and press to form a positive electrode material layer with a thickness of 80 μm on each side. Then, Comparative Examples 1A and 1B and Examples 1A to 1E and Comparative Examples 2A to 2C and Examples 2A to 2D shown in Tables 1 and 2 were prepared. Tables 1 and 2 show the presence or absence of fluidity of the positive electrode material slurry at the time of manufacturing each positive electrode plate.
[0017]
[Table 1]

[Table 2]

In Comparative Examples 1A, 1B, 2A, and 2B where the pH value of the dispersion exceeds 11.2 from Tables 1 and 2, a positive electrode plate that does not have the fluidity of the positive electrode material slurry and functions effectively as a battery cannot be formed. It was.
[0018]
Next, a test lithium ion secondary battery shown in FIG. 1 was made using each positive electrode plate except Comparative Examples 1A, 1B, 2A, and 2B. As shown in the figure, the lithium ion secondary battery has a structure in which a wound electrode group 1 is housed in a battery can 2. The wound electrode plate group 1 has a structure in which the positive electrode plate 3 and the negative electrode plate 4 are wound so as to be laminated via an electrolyte layer (separator) 5. The positive electrode plate 3 has a structure in which positive electrode material layers 7 are formed on both surfaces of a positive electrode current collector 6. The negative electrode plate 4 has a structure in which a negative electrode material layer 9 is formed on both surfaces of a negative electrode current collector 8. The lithium ion secondary battery was manufactured as follows. First, the negative electrode plate 4 was manufactured. First, 90% by weight of a negative electrode material made of graphite carbon material having an average particle size of 15 μm and 10% by weight of a binder made of polyvinylidene fluoride were mixed. Note that amorphous carbon may be used as the negative electrode material. An appropriate amount of a solvent composed of N-methyl-2-pyrrolidone (NMP) was added thereto and kneaded sufficiently to prepare a negative electrode slurry. Next, a negative electrode material slurry was applied to both surfaces of a negative electrode current collector 8 made of a strip-shaped copper foil of 10 μm × 50 mm × 490 mm by roll-to-roll transfer, dried and pressed, and negative electrode material layer 9 having a thickness of 105 μm on each surface The negative plate 4 was made. The negative electrode material layer 9 has a density of 1.3 to 1.45 g / cm ³ .
[0019]
Next, each of the positive electrode plate 3 and the negative electrode plate 4 described above was wound through a strip-shaped separator 5 made of a polyethylene microporous film having a thickness of 25 μm to form an electrode plate group 1. Next, after the electrode plate group 1 is disposed in the cylindrical battery can 2 made of Ni-plated iron, the nickel tab terminal 11 that is previously welded to the negative electrode current collector 8 is welded to the bottom 2 a of the battery can 2. did. Next, an organic electrolytic solution (nonaqueous electrolytic solution) in which a lithium salt composed of LiPF ₆ was dissolved at a concentration of 1 mol / l in a solvent obtained by mixing ethylene carbonate, dimethyl carbonate, and diethyl carbonate in a volume ratio of 30:50:20. 5 ml was injected into the battery can 2. Next, the aluminum tab terminal 10 previously welded to the positive electrode current collector 6 was welded to a battery lid 12 having a current interrupt mechanism (pressure switch) and a valve (not shown). This valve opens at a pressure higher than the pressure at which the current interrupt mechanism (pressure switch) operates. Then, after the battery lid 12 is disposed on the upper part of the battery can 2 via the gasket 13 made of insulating polypropylene, the inside of the battery can 2 is sealed to seal each cylindrical lithium ion secondary battery. Had made.
[0020]
Next, each lithium ion secondary battery was charged at a low voltage of 4.21 V (limited current of 320 mAh) in an atmosphere of 25 ° C. for 8 hours, and then discharged to a final voltage of 2.5 V at 1.6 A to obtain a positive electrode material ( The discharge capacity per gram of lithium nickel composite oxide) was determined. Tables 1 and 2 show the discharge capacities of batteries made using each positive electrode plate. From Table 2, it can be seen that in Comparative Example 2C, the pH value of which is lower than 7.1, the active material is dissolved in the electrolytic solution, so that the discharge capacity is lowered.
[0021]
Next, as shown in Table 3, the lithium nickel composite oxides having different pH values exceeding 11.2 were obtained by changing the stirring time of the dispersion or the amount of phosphoric acid added. Then, using each lithium nickel composite oxide, a positive electrode material slurry was prepared in an atmosphere of 3% RH by a dehumidifier, and a lithium ion secondary battery was prepared. The positive electrode material slurry and the lithium ion secondary battery were manufactured under the same conditions as in the above test except for the atmospheric humidity. The presence or absence of fluidity of the positive electrode material slurry and the discharge capacity per gram of the positive electrode material (lithium nickel composite oxide) of the lithium ion secondary battery were examined. Table 3 shows the measurement results.
[0022]
[Table 3]

From Table 3, it can be seen that each positive electrode slurry prepared in an atmosphere of 3% RH has fluidity. However, it can be seen that batteries manufactured using each positive electrode material slurry have a low discharge capacity at a high load of 154 to 158 mAh / g, and a sufficient capacity cannot be obtained. Therefore, it can be understood that the discharge capacity at the time of high load remains low even if a dehumidifying atmosphere measure is taken when the positive electrode material slurry is generated.
[0023]
【The invention's effect】
According to the present invention, since the positive electrode material slurry is made using a lithium nickel composite oxide that does not become strong alkaline even when dispersed in water, even if the positive electrode material slurry contains a small amount of water, the positive electrode material slurry is It becomes almost neutral or weakly alkaline, and the three-dimensional binder can be suppressed to prevent the positive electrode material slurry from gelling.
[Brief description of the drawings]
FIG. 1 is an end view of a lithium ion secondary battery manufactured by a method according to an embodiment of the present invention.
[Explanation of symbols]
1 Winding type electrode plate group 2 Battery can 3 Positive electrode plate 4 Negative electrode plate 5 Electrolyte layer (separator)
6 Positive electrode current collector 7 Positive electrode material layer 8 Negative electrode current collector 9 Negative electrode material layer

Claims

A positive electrode plate for a lithium ion secondary battery is manufactured by applying a slurry containing a lithium nickel composite oxide containing at least lithium element and nickel element as main components, a binder, and an organic solvent on a current collector. In the way to
Lithium nickel composite oxide material containing Li ₂ O and lithium nickel composite oxide is dispersed in water by 8 to 12% by weight to make a dispersion,
Stirring the dispersion until the pH of the dispersion is 7.1-11.2,
The dispersion is then filtered to remove the residue,
A method for producing a positive electrode plate for a lithium ion secondary battery , comprising using the dried residue as the positive electrode material .

A positive electrode plate for a lithium ion secondary battery is manufactured by applying a slurry containing a lithium nickel composite oxide containing at least lithium element and nickel element as main components, a binder, and an organic solvent on a current collector. In the way to
Lithium nickel composite oxide material containing Li ₂ O and lithium nickel composite oxide is dispersed in water by 8 to 12% by weight to make a dispersion,
Add phosphoric acid to the dispersion until the pH of the dispersion is 7.1-11.2,
The dispersion is then filtered to remove the residue,
A method for producing a positive electrode plate for a lithium ion secondary battery , comprising using the dried residue as the positive electrode material .

The method for producing a positive electrode plate for a lithium ion secondary battery according to claim 1 or 2, wherein polyvinylidene fluoride is used as the binder, and N-methyl-2-pyrrolidone is used as the organic solvent .